1. Field of the Invention
The present invention relates to sheet metal manufacturing processes that include trimming and flanging sheet metal parts.
2. Background Art
Sheet metal manufacturing processes are used to manufacture interior panels, exterior panels and other structural parts of manufactured products. Sheet metal manufacturing processes originally focused on manufacturing parts from mild steel. Mild steel is a preferred material for sheet metal manufacturing processes because of its favorable material properties that allow parts to be drawn, formed, flanged, trimmed and welded relatively easily.
In an effort to increase strength and reduce the weight of some sheet metal parts, such as those used in the manufacture of vehicles, considerable effort has been devoted to using light-weight, high performance materials. Examples of such light-weight materials include Bakehardenable Steels, Dual Phase Steels, Boron Steels and high strength aluminum alloys. Unfavorable material properties of these high strength, light-weight materials pose a wide variety of technological problems for conventional sheet metal forming processes.
One problem area encountered when using high strength, light-weight materials in sheet metal forming processes relates to the trimming operation. Trimming, as used herein, refers to shearing off extra material from a drawn panel. In many instances, especially when the geometry of a part being trimmed is rather complicated, it may be necessary to perform two trimming operations. Conventional trimming dies usually incorporate an upper trim steel and a lower trim steel that shear extra material from the drawn panel while the panel is retained on the lower die by a clamping pad that holds the panel against the lower trim steel cutting member.
Advanced high strength steels require applying substantially greater forces to the trimming dies. In conventional trimming dies, clearance between the upper trim steel and lower trim steel should be less than 10% of the material thickness. However, with advanced high strength steel and aluminum, visible burrs may be formed which typically indicates that the die has excessive clearance. Burrs may form even if the die is manufactured to the normal clearance of less than 10% of the blank thickness if the die has insufficient stiffness against expanding the clearance between shearing edges driven by trimming forces. Trimming advanced high strength steels and aluminum may also result in increased die wear and, more specifically, increased wear of the trimming edges of the upper and lower trim steels. Wear may also add to increased clearance, decreased sharpness and other problems relating to the trim operation.
Another problem relating to sheet metal forming operations on less ductile advanced high strength steel and aluminum alloys relates to the reduced formability as it impacts flanging operations. Splits emanating from a trimmed surface may be created during a stretch flanging operation. Stretch flanging operations can also be adversely impacted by reduced sharpness of trim die tool steel. To address this issue, it has been proposed to combine trimming operations and one flanging operation in one electro-hydraulic forming (EHF) tool. However, this approach does not eliminate the issue of stretch flange splitting especially after the trim steels become worn, splits may subsequently occur in the flanging operation. Another problem encountered when the panels are trimmed and then flanged may include generation of wrinkles in transition zones and waviness of the flange. Transition zones may be created when sequential discharges of the EHF tool are required to trim and flange the part.
The trim die may require a cam-driven trimming tool if a large cutting angle is required. Cam-driven trimming tools make the trim die more expensive and require additional maintenance. Trim dies must be maintained by resharpening the trim steel edges or by compensating for wear by shimming the trim steels. The trim die may require adjustment to prevent or eliminate splits created in stretch flanging operations. Trimmed parts may require additional metal finishing if the material forms slivers or localized splits created along a trimmed edge during subsequent stretch flanging operations.
These and other problems are addressed by Applicant's development of an improved sheet metal manufacturing process.